1. Field of the Invention
This invention relates to a process for separating the low boiling components from a gaseous mixture of hydrogen and hydrocarbons, and in particular, to a process for demethanization in which a high recovery of relatively pure stream of hydrogen off-gas is obtained.
2. Description of the Prior Art
Ethylene is an important chemical in the petrochemical industry, particularly in the production of polymers. Ethylene may be obtained through separation processes from hydrocarbon mixtures which are derived from various sources such as normal refinery operations, cracking processes and the like. The content of the hydrocarbon mixture containing ethylene will vary depending upon the source of the hydrocarbon mixture. However, it is typical that a hydrocarbon mixture used in an ethylene production plant will include hydrogen, methane, ethane, ethylene and other higher hydrocarbons.
In general, the process of removing the methane and the hydrogen, referred to as demethanization, will usually be the single most expensive step in separating ethylene from the original hydrocarbon feed mixture. The expense associated with demethanization is a result of the low temperature requirement of this step and the vapor liquid equilibrium of the methane-ethylene system. A good discussion of demethanization as well as other processes utilized in the complete process of separating ethylene from an initial hydrocarbon feed mixture is given in King, The Low Temperature Separation of Hydrocarbons, Trans. Instn Chem. Engrs., Volume 36, page 162, 1958, the disclosure of which is hereby specifically incorporated by reference. Although steps such as depropanization and ethylene-ethane separation will be required before ethylene can be separated from typical hydrocarbon feed mixtures, this invention is primarily concerned with demethanization.
During all demethanization processes, hydrogen and methane must be removed from the hydrocarbon gas mixture. Often, there is economic incentive to recover as much of the hydrogen as possible as a relatively pure stream of at least 90% purity. This hydrogen stream can then be utilized as a chemical feed stock for other processes.
In early designs, simple demethanizers were used in which the total hydrocarbon mixture was chilled and fed to the demethanizer. This is the type of system described by King in the above reference. Such a system featured high ethylene losses because all the hydrogen flowed through the demethanizer. The ethylene loss is set by the dew point temperature that can be achieved by the overhead. To recover some of the ethylene, some of the demethanizer overhead was recycled through the plant, thus increasing equipment sizes and energy consumption.
Modern designs use a separator at each stage of chilling. The bottoms from each chilling stage separator is fed into a multifeed demethanizer. The uncondensed overheads stream from the last chilling stage contains most of the hydrogen in the initial hydrocarbon mixture. This hydrogen-rich gas bypasses the demethanizer, thus improving ethylene recovery and eliminating the need for plant recycles.
This system lends itself to hydrogen recovery from the hydrogen-rich gases. However, even with this system, some hydrogen still enters the demethanizer, ends up in the demethanizer overheads stream and is difficult to recover. In an ethylene plant using ethane feedstock, the demethanizer overheads stream typically contains around 10 mole percent hydrogen.
To obtain a high-purity hydrogen stream, the hydrogen-rich stream from the final feed chilling stage is cooled and sent to the hydrogen separator which operates at a temperature typically lower than -140.degree. C. To obtain this low temperature, the methane-rich stream from the demethanizer overheads is expanded to a low pressure, thus generating the "deep cold" conditions required for high-purity hydrogen separation. When it is economical to achieve a high recovery of hydrogen, a significant portion of the methane-rich stream needs to be condensed to provide the required low temperature refrigeration.
The recovery of hydrogen from a demethanization system is limited by several factors. First, some hydrogen enters the demethanizer and ends up in the methane-rich stream. Since this stream contains only a small quantity of hydrogen, and since it is collected at low pressure, it is usually difficult and uneconomical to recover hydrogen from it. Second, the presence of hydrogen in the methane-rich stream as it comes off the demethanizer hinders condensation of this stream. This limits the amount of "cold" that can be obtained from expanding the methane-rich stream. Furthermore, the low vapor pressure of methane at the low temperatures needed for the hydrogen-methane separation requires that the methane liquid be vaporized at a low partial pressure in order for the methane-rich stream to provide the required refrigeration. This low partial pressure is provided by bleeding significant quantities of hydrogen-rich gas into the methane-rich stream.
The above factors reduce the quantity of hydrogen that can economically be recovered from the demethanization step. This problem is most severe when the ratio of hydrogen to methane in the process gas mixture is high.
Three conventional alternative methods may be utilized to enhance hydrogen recovery. All three increase the availability of "cold" required for the hydrogen separation, thereby reducing the quantity of hydrogen bleed required, but none of these recovers the hydrogen from the methane rich gas. In an ethylene plant cracking ethane feedstock, the hydrogen lost in the methane-rich gas is typically 4% of the total hydrogen in the process gas mixture. In addition, all these three methods have disadvantages, as described below.
One method utilizes an external methane refrigeration circuit. Such a system is expensive, both in capital and operating costs.
A second method compresses the methane-rich stream leaving the demethanizer overheads to a relatively high pressure, approximately 45 bars. This method involves a large compressor which utilizes a substantial amount of power and heats the compressed demethanizer overheads stream. The resultant heat must then be removed by the coldest level of ethylene refrigeration. An additional problem with this system is the difficulty in condensing reflux for the demethanizer. When insufficient reflux is available, there is an excessive loss of ethylene from the top of the demethanizer column, unless some additional reflux is condensed and separated at the overhead compressor discharge. Condensing reflux at the compressor discharge depends on a separation at temperatures and pressures which approach the critical too closely. Accordingly, such separation cannot be predicted with confidence.
A third method involves recycling expanded liquid back into the demethanizer. Recycling this stream represents a waste of energy and requires the use of cold pumps, which are an operating nuisance. By itself, this method can only provide a limited amount of cold, but it can be used in combination with the second method above.
Thus, there exists a need for a process of removing hydrogen and methane from a gaseous hydrocarbon mixture in which a high recovery of a relatively pure stream hydrogen is economically obtained without the need to utilize an external methane refrigeration circuit or the need to compress the methane-rich stream to high pressures.